The test can check for cancer DNA mutations and also monitor how fast treatment is working

By looking for cancer DNA in blood samples, researchers were able to identify men with defective BRCA genes who were likely to benefit from drugs called PARP inhibitors.

The test can also be used to monitor DNA in the blood after treatment had started, allowing patients who were not responding quickly to be switched to alternative treatments.

The same test was also used to pick up signs of evolving cancer showing the first signs of drug resistance.

Professor Johann de Bono, who led the team at The Institute of Cancer Research in London, said: “We think it could be used to make clinical decisions about whether a PARP inhibitor is working within as little as four to eight weeks of starting therapy.

“Not only could the test have a major impact on treatment of prostate cancer, but it could also be adapted to open up the possibility of precision medicine to patients with other types of cancer as well.”

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More than 40,000 men in the UK are diagnosed with prostate cancer every year, making it the most common male cancer.

Around 11,000 die from the disease, which is often localised, slow-growing, and relatively ­simple to treat.

The test could see a PARP inhibitor, called olaparib, become a standard treatment for advanced prostate cancer.

PARP inhibitors such as olaparib block an enzyme used by cancer cells with defective BRCA 1 and 2 genes to repair their DNA.

When this enzyme is blocked the cells die.

While some patients respond to the drugs for years, others either fail to respond at an early stage or develop resistant cancer.

“They are cheap and simple to use, but most importantly, because they aren’t invasive, they can be employed or applied to routinely monitor patients to spot early if treatment is failing – offering patients the best chance of surviving their disease.

Scientists hope the new test will pave the way for more personalised treatment for advanced prostate cancer patients

“This test is particularly exciting because it is multi-purpose, designed for use both before and after treatment, and using both the absolute amounts of cancer DNA in the bloodstream and also a readout of the specific mutations within that genetic material.”

Dr Matthew Hobbs, deputy director of research at the charity Prostate Cancer UK, which funded the research, said: “It’s clear that we need to move away from the current one-size-fits-all approach to much more targeted treatment methods.

“The results from this study and others like it are crucial as they give an important understanding of the factors that drive certain prostate cancers, or make them vulnerable to specific treatments.”